Integrated circuits are made up of literally millions of active devices such as transistors and capacitors. These devices are initially isolated from each other, and are later interconnected to form functional circuits. Typical interconnect structures include lateral interconnections, such as metal lines (wirings), and vertical interconnections, such as vias and contacts. Interconnect structures are increasingly determining the limits of performance and the density of modern integrated circuits.
On top of the interconnect structures, connector structures are formed. The connector structures may include solder balls or metal pillars that are exposed on the surface of the respective chip. Electrical connections are made through the solder balls or the metal pillars to connect the chip to a package substrate or another die.